1. Field of the Invention
The present invention relates to a nitride-based semiconductor device having a substrate formed of a ZnO single crystal and a nitride-based semiconductor layer formed and grown on the substrate, and a manufacturing method of the same.
2. Description of the Related Art
Conventionally, a semiconductor light emitting device using InGaN is known as a semiconductor light emitting device emitting blue light (a wavelength of not longer than 480 nm) (refer to Patent Reference 1).
In the semiconductor light emitting device using InGaN, an In content increases to reduce a band gap of an active layer for emitting green light. However, when the In content increases, a phase separation tends to occur, thereby making it difficult to obtain the active layer having a uniform In content and decreasing light emitting efficiency. In addition, when a piezoelectric field occurs due to a crystal structure, a probability of radiative recombination decreases, thereby further decreasing the light emitting efficiency. Furthermore, a large amount of threading dislocations occur due to a large lattice constant mismatch relative to the substrate, thereby decreasing the light emitting efficiency and reliability.
To this end, there has been proposed a substrate lattice-matching to the active layer of the nitride-based semiconductor such as InGaN and the like, or a cladding layer having a lattice constant similar to that of the InGaN active layer. The substrate is preferably formed of a ZnO single crystal.
However, when the InGaN layer is directly grown on the substrate formed of the ZnO single crystal, the substrate reacts with the InGaN layer at an interface thereof, thereby making it difficult to obtain a steep interface and a good quality crystal. More specifically, Ga diffuses into the substrate so that it is difficult to obtain the steep interface; and a reaction layer (Ga2ZnO4) is formed at the interface between the substrate and the InGaN layer, thereby making it difficult to obtain the InGaN layer with good crystal quality. When the InGaN layer is grown at a low temperature, it is possible to suppress the diffusion of Ga and the formation of the reaction layer (Ga2ZnO4). However, crystal defects tend to be generated during the low temperature growth, thereby decreasing crystallinity of the InGaN layer (refer to Non-patent Reference 1).
Another technology has been known in which, after amorphous Al2O3 is deposited on a substrate at a low temperature (100° C.) with an atomic layer deposition (ALD) method, amorphous Al2O3 is crystallized through a high temperature thermal treatment (1,100° C., 20 to 40 minutes), and then an InGaN layer is grown on the crystallized Al2O3 layer with a MOCVD method (refer to Non-patent reference 2).    [Patent Reference 1]
Japanese Patent Publication No. 06-061527    [Non-patent Reference 1]
MRS Internet Journal vol. 1, Article 16, 1996    [Non-patent Reference 2]
MRS Symp. Proc. Vol. 1035, L 11-23, 2007 Fall Meeting Nov. 26-30
In the conventional technology disclosed in Non-patent Reference 2, the Al2O3 layer formed on the substrate has a large thickness of 20 to 50 nm. Therefore, it is difficult to transmit a wurtzite type crystal structure and a lattice constant of the ZnO single crystal substrate to the InGaN layer at an upper portion, thereby making it difficult to obtain the InGaN layer with good crystal quality. Furthermore, in the Non-patent Reference 2, the reaction layer (Al2ZnO4) is formed at the interface between the substrate and the InGaN layer, thereby causing the InGaN layer with insufficient crystal quality.